This has happened 3 times now over a period of a few months (yes I know I need to get it to a mechanic ASAP - I shall do it this week). At first I wasn't sure what was causing it but it happened again today while stuck in traffic (like the other times), just after the fans came on, so it must be to do with the car overheating. The whining/groaning has stopped after a few seconds each time. I was expecting the STOP light to come on but it never has. I've checked the coolant level and it seems fine so I don't know what else to look at. I know the mechanic will be able to sort it for me but I'd like to know in advance.

I said the whining started AFTER the fans came on so the noise is not the fans..... the noise is not a normal noise - there is a problem